How to straighten chisel edge?

D

DavidL41

Joined
Oct 12, 2014
Messages
160
How would you sharpen a chisel that isn't straight right at its edge - if i placed only the edge touching a flat bench light would show right under the edge.

At this point would the only way to straighten that edge be by grinding the edge 90 degrees it till its flat, then progress from low grit to medium?

I also have a similar issue where the bevel in some spots are 9-10 degrees and the other end is 11-12 degrees (judging from the shoulder of the bevel not being straight). How would I take off that high spot down from 11-12 degrees to 9-10 degrees? My method that I "tried" was to lift up the end with the 9-10 degree bevel so that it doesnt touch the sandpaper, sanding down the spot that is 11-12 degrees. I believe that this will create some unevenness unless I somehow am able to flatten the high spots and low spots till its level.
 
If I get this right, light shines through some points on the edge when placed edge down, dead vertical?

I hate to say it but if you need it dead on 90° you will have to flatten it across the edge and work it back. This will also give you a chance to clean up the degrees on the bevel side. Is this an actual chisel or a chisel edge on a sport knife?

When doing these I stop often to see where the grind is developing - minute shifts of finger contact with the off hand can correct any gradual drifting - very easy to have happen with chisels when free-handing due to small contact area and tall handle relative to that contact area. If comfortable for you, grip it further down, closer to the edge itself. I also work it at a diagonal to keep any wooble to a minimum. Use a Sharpie often if needed and observe often.

If a small difference is being corrected you can usually just shift the finger placement to increase the amount of force being applied to the thicker portion and it will correct itself with some additional grinding. Unless the edge is very off or needs to be 100% accurate for inletting etc, I wouldn't worry about correcting it (the 90° issue). Another consideration is that when working with sandpaper there will always be some convexing of the edge - the corners will wear faster than the center, creating a minute "smile" - how much will depend on the thickness of your sandpaper, the surface underneath, and how much force is being applied. This can be reduced a great deal once one is aware of it, trust in Sharpie.
 
Yes, it is a chisel for leather working, same blade shape as a wood chisel.

The exact way you described the convexing I have with my chisel. What reason does sandpaper have that creates that convexing of the edge, should I apply nearly no pressure on the left and right ends of the blade? Is this an issue I will have with a waterstone or dmt?
 
DavidL41 said:
Yes, it is a chisel for leather working, same blade shape as a wood chisel.

The exact way you described the convexing I have with my chisel. What reason does sandpaper have that creates that convexing of the edge, should I apply nearly no pressure on the left and right ends of the blade? Is this an issue I will have with a waterstone or dmt?
Click to expand...

The harder waterstones, ceramics, DMT, Arkansas will be the highest level of precision.

All waterstones will wear grooves and hollows that can effect flatness, but as long as you move around on the stone, rotate it as you go, will still produce a near perfect flat without need for constant lapping. You have to avoid hitting the same spot on the stone with the same pass every time. This will keep it from deforming the surface too bad in any one spot, and prevent the deformations that do crop up from influencing the outcome. Key - do not wait until the stone has notable dishes and then try to correct after the fact by working it - at that stage you will need to lap it flat. Use the entire surface as a routine action and try to do so on every abrasive surface, not just waterstones.

Softer waterstones, vitreous stones that break down - anything that creates mud as it grinds will be the next step down. As you climb into the higher grit values this will be less and less of a factor. Refer to above about moving around on the stone. I also believe sandpaper/thin papers over my Washboard fall in this range based on micrographs I have taken and by testing going back and forth between diamond plate and other means.

Sandpaper over hard surface will have the least overall relative precision due to the give of the paper - it collapses a bit at pressure levels below what would be influenced by the backing, though softer backings will definitely increase this factor there is a limit to how much help the harder backings will provide. Use the wet/dry with the thinnest substrate/paper backing you can find. Keep in mind with a light touch this can be greatly minimized and along the cutting edge can be almost indistinguishable from other means, the more broad the bevel/surface area the easier to minimize. The outside corners are where it will be most evident and virtually impossible to make dead flat, though plenty good enough for most work and again, not really evident in use or casual visual inspection.

I have had some success by sharpening as I always would, then going back and working the center of the edge on the outside edge of the block to bring it more in line with the outside corners, but this is not a preferred method. If you want the absolute flattest, avoid sandpaper entirely.

Martin
 
I'm sure this is an expensive chisel you're talking about, as opposed to a $10 or under chisel that I'm used to. I've never sharpened a nice one. That being said, my shop teacher taught us all how to sharpen a damaged chisel many years ago in shop class. The process seems applicable, at least in part to your situation.

1. Grind the cutting edge flat and square to the body of the chisel. Meaning, take out any chips or dings in the cutting edge by grinding until they disappear. The "square" part means to keep the cutting edge 90 degrees to the chisel body.
2. Regrind the hollow of the chisel until it meets the flat back side. We did this by placing the hollow ground front of the chisel directly on the rounded part of a powered grinding wheel. Holding at the bottom of the wheel is safe(r) because it's moving away from you and should not be able to "throw" the tool. We continued grinding until the hollow of the chisel had been extended to the cutting edge. This is what you want so you can continue to have a thin profile near the cutting edge for future sharpening. Note that this can remove metal quickly and can make the tool very hot and ruin it's heat treat. Stop and dip in water frequently to keep the heat to a reasonable level.
3. Finally, we're going to actually sharpen it. Place the chisel so that the cutting edge is on the stone at an angle that's proper for sharpening. A guide for this is to put the chisel flat until the thicker part of the chisel body touches the stone also. It's kind of a built in angle guide if you use it this way. Grind until you get a burr on the flat back side.
4. Remove the burr from the flat back side with flat strokes on the stone.

My teacher would then (to show off) polish the cutting edge on a powered cotton wheel loaded with red rouge. The produced an edge that would cleanly mow hair right off of his hairy arm.

This may seem crude or like too much metal removal. This procedure is designed to recondition a chisel that's so dull that a regular sharpening won't make it perform well. This includes dings, nicks, etc, kind of like the condition you are describing of being able to see light under the cutting edge.

Step #2 above is the most difficult in terms of keeping it consistent and visually pleasing. You have to keep the angle constant *and* move the chisel back and forth continuously in order to grind in a uniform hollow ground face. Finding the right angle can be tricky too. I'm not good at this at all.

Good luck to you.

Brian.
 
There are LOTS of good reference videos on YouTube for sharpening chisels & plane irons, but I just finished watching this one. It's a pretty good demo of how to flatten/polish/shape both the back and the bevel (he re-shapes the brand new factory bevel to a cambered/convex face), using both wet/dry paper (on glass) and diamond plates, as well as stropping on wood and leather. He uses the wet/dry paper mainly for flattening/shaping/polishing, then adds a few passes on diamond plates (thru 1200) to hone the cutting edge, followed by stropping on what appears to be white compound (he says he favors either/both of chrome oxide and aluminum oxide for stropping; appears he's using the AlOx). Follows with a definitive demo of sharpness as well, both push-cutting paper and removing large slices of material from a pine board.

There's some good reason to glue or otherwise adhere the paper to a hard surface, as it's evident what happens if the paper is allowed to move at all (he rips/cuts a couple pieces).

[video=youtube;Ki8tt-VjwqI]https://www.youtube.com/watch?v=Ki8tt-VjwqI[/video]


David
 
Last edited:
You must log in or register to reply here.
Back
Top